1. Field of the Invention
The present invention relates to techniques for semiconductor device manufacturing. More specifically, the present invention relates to a method for fabricating a substrate with at lest one high-quality surface for growing semiconductor structures.
2. Related Art
Semiconductor device manufacturing relies on polished mirror-like substrates as bases or supporting structure to grow layers of thin-film materials and semiconductor structures. Typically, one or both surfaces of a silicon (Si) single-crystal substrate can be polished to a roughness less than a few nanometers. Furthermore, the surfaces of these Si substrates can be prepared to exhibit specific crystal orientations, which are preferable to the subsequent epitaxial growth of semiconductor structures.
Recently, increasing research activities have been focused on using metals or alloys as substrate materials. In comparison with conventional non-metal substrates, metal substrates possess some superior properties, which include excellent thermal conductivity, electrical conductivity, and optical reflectivity. These metal substrates can be used for semiconductor thin film epitaxial growth, as well as for hermetic wafer bonding.
However, metal substrates have had relatively limited applications in semiconductor device manufacturing. This limitation is primarily due to the fact that metal substrates typically have inferior surface qualities, including insufficient smoothness and flatness, in comparison with conventional non-metal substrates, such as Si and germanium (Ge) substrates.
Currently, fabricating metal substrates typically requires using one or more polishing techniques, which include mechanical polishing, chemical polishing, electric polishing, or a combination of the above. For example, U.S. Pat. No. 6,867,447 discloses a method for preparing a metal substrate surface using electromechanical polishing, and PCT/US2002/041453 describes a technique for polishing metal substrate using a chemical-mechanical polishing (CMP) process. Unfortunately, because of the plasticity and ductility associated with metallic materials, it is extremely difficult to adequately polish these metal substrates using the existing polishing techniques to achieve the technical specifications equivalent to those of Si or Ge substrates. Moreover, these metal-polishing techniques suffer from greater complexity, lack of repeatability, higher cost, and are prone to causing contamination on the polished metal surfaces.
Another serious disadvantage associated with polished metal substrates relates to a general lack of crystalline structure of bulk metal materials. Metal substrates are typically obtained from cutting smelted bulk metallic materials into thin slices and polishing them. However, these bulk metallic materials are comprised of predominately microscopic polycrystals which have random crystalline orientations. Hence, the resulting surfaces do not have a well-defined crystalline orientation and are unsuitable for epitaxial growth of semiconductor structures, even if these substrates have perfectly polished surfaces.
Hence, what is needed is a method and an apparatus for preparing high-quality metal substrates suitable for fabricating semiconductor structures without the above-described problems.